On Tue, Apr 26, 2016 at 4:17 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, 26 Apr 2016 09:21:10 -0700 Thomas Garnier <thgarnie@xxxxxxxxxx> wrote: > >> Provides an optional config (CONFIG_FREELIST_RANDOM) to randomize the >> SLAB freelist. The list is randomized during initialization of a new set >> of pages. The order on different freelist sizes is pre-computed at boot >> for performance. Each kmem_cache has its own randomized freelist. Before >> pre-computed lists are available freelists are generated >> dynamically. This security feature reduces the predictability of the >> kernel SLAB allocator against heap overflows rendering attacks much less >> stable. >> >> For example this attack against SLUB (also applicable against SLAB) >> would be affected: >> https://jon.oberheide.org/blog/2010/09/10/linux-kernel-can-slub-overflow/ >> >> Also, since v4.6 the freelist was moved at the end of the SLAB. It means >> a controllable heap is opened to new attacks not yet publicly discussed. >> A kernel heap overflow can be transformed to multiple use-after-free. >> This feature makes this type of attack harder too. >> >> To generate entropy, we use get_random_bytes_arch because 0 bits of >> entropy is available in the boot stage. In the worse case this function >> will fallback to the get_random_bytes sub API. We also generate a shift >> random number to shift pre-computed freelist for each new set of pages. >> >> The config option name is not specific to the SLAB as this approach will >> be extended to other allocators like SLUB. >> >> Performance results highlighted no major changes: >> >> Hackbench (running 90 10 times): >> >> Before average: 0.0698 >> After average: 0.0663 (-5.01%) >> >> slab_test 1 run on boot. Difference only seen on the 2048 size test >> being the worse case scenario covered by freelist randomization. New >> slab pages are constantly being created on the 10000 allocations. >> Variance should be mainly due to getting new pages every few >> allocations. >> >> ... >> >> --- a/include/linux/slab_def.h >> +++ b/include/linux/slab_def.h >> @@ -80,6 +80,10 @@ struct kmem_cache { >> struct kasan_cache kasan_info; >> #endif >> >> +#ifdef CONFIG_FREELIST_RANDOM >> + void *random_seq; >> +#endif >> + >> struct kmem_cache_node *node[MAX_NUMNODES]; >> }; >> >> diff --git a/init/Kconfig b/init/Kconfig >> index 0c66640..73453d0 100644 >> --- a/init/Kconfig >> +++ b/init/Kconfig >> @@ -1742,6 +1742,15 @@ config SLOB >> >> endchoice >> >> +config FREELIST_RANDOM >> + default n >> + depends on SLAB >> + bool "SLAB freelist randomization" >> + help >> + Randomizes the freelist order used on creating new SLABs. This >> + security feature reduces the predictability of the kernel slab >> + allocator against heap overflows. > > Against the v2 patch I didst observe: > > : CONFIG_FREELIST_RANDOM bugs me a bit - "freelist" is so vague. > : CONFIG_SLAB_FREELIST_RANDOM would be better. I mean, what Kconfig > : identifier could be used for implementing randomisation in > : slub/slob/etc once CONFIG_FREELIST_RANDOM is used up? > > but this pearl appeared to pass unnoticed. > It was discussed a bit before. The intent is to have a similar feature for other kernel heap (I know it is possible for SLUB). That's why I think it make sense to have a similar config name used for all allocators. >> config SLUB_CPU_PARTIAL >> default y >> depends on SLUB && SMP >> diff --git a/mm/slab.c b/mm/slab.c >> index b82ee6b..0ed728a 100644 >> --- a/mm/slab.c >> +++ b/mm/slab.c >> @@ -1230,6 +1230,61 @@ static void __init set_up_node(struct kmem_cache *cachep, int index) >> } >> } >> >> +#ifdef CONFIG_FREELIST_RANDOM >> +static void freelist_randomize(struct rnd_state *state, freelist_idx_t *list, >> + size_t count) >> +{ >> + size_t i; >> + unsigned int rand; >> + >> + for (i = 0; i < count; i++) >> + list[i] = i; >> + >> + /* Fisher-Yates shuffle */ >> + for (i = count - 1; i > 0; i--) { >> + rand = prandom_u32_state(state); >> + rand %= (i + 1); >> + swap(list[i], list[rand]); >> + } >> +} >> + >> +/* Create a random sequence per cache */ >> +static int cache_random_seq_create(struct kmem_cache *cachep) >> +{ >> + unsigned int seed, count = cachep->num; >> + struct rnd_state state; >> + >> + if (count < 2) >> + return 0; >> + >> + /* If it fails, we will just use the global lists */ >> + cachep->random_seq = kcalloc(count, sizeof(freelist_idx_t), GFP_KERNEL); >> + if (!cachep->random_seq) >> + return -ENOMEM; > > OK, no BUG. If this happens, kmem_cache_init_late() will go BUG > instead ;) > Yes, as Christophe asked. > Questions for slab maintainers: > > What's going on with the gfp_flags in there? kmem_cache_init_late() > passes GFP_NOWAIT into enable_cpucache(). > > a) why the heck does it do that? It's __init code! > > b) if there's a legit reason then your new cache_random_seq_create() > should be getting its gfp_t from its caller, rather than blindly > assuming GFP_KERNEL. > > c) kmem_cache_init_late() goes BUG on ENOMEM. Generally that's OK in > __init code: we assume infinite memory during bootup. But it's really > quite weird to use GFP_NOWAIT and then to go BUG if GFP_NOWAIT had its > predictable outcome (ie: failure). > > Finally, all callers of enable_cpucache() (and hence of > cache_random_seq_create()) are __init, so we're unnecessarily bloating > up vmlinux. Could someone please take a look at this as a separate > thing? > >> + /* Get best entropy at this stage */ >> + get_random_bytes_arch(&seed, sizeof(seed)); >> + prandom_seed_state(&state, seed); >> + >> + freelist_randomize(&state, cachep->random_seq, count); >> + return 0; >> +} >> + >> >> ... >> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>